1. Field of the Invention
The present invention relates to a fixed probe board which is useful for testing semiconductor devices formed on semiconductor wafers.
2. Description of the Prior Art
Among various procedural steps involved in the production of a semiconductor such as a monolithic semiconductor integrated circuit, the probing step has been regarded as a mere intermediate step of testing or inspection. However, when the entire process is generally divided into two procedures, i.e. one including steps up to the production of a masked wafer and the other including subsequent steps up to the production of a final product, the probing step constitutes a final inspection step in the procedure for the production of a wafer and thus plays an extremely important role in a sense that it gives a great influence over the labour-saving and work efficiency in the subsequent steps. Namely, automation of the subsequent steps has made it possible to produce final products with minimal production of defective products, and under the circumstances, the accuracy or precision of the inspection in the probing step has become a decisive factor for the yield of final products.
A fixed probe board is used in such a probing step. A typical fixed probe board is disclosed, for instance, in an article entitled "Use of Fixed Probe Card" in Electronic Materials (Special Edition of November 1970 issue) p. 145, published on Nov. 5, 1970, and in U.S. Pat. No. 3,835,381 to Oliver R. Garretson et al. In the above mentioned article, usefulness of the fixed probe board is described such that it has overcome a drawback of an adjustable multipoint probe i.e. poor operability for alignment or maintenance of the probes for mass production. The construction of this fixed probe board is disclosed in detail in the above mentioned U.S. Patent. This probe board (i.e. a test probe assembly) comprises a multiplicity of probes fixed by a support base and an adhesive layer in a conical radial array relative to a printed substrate.
Semiconductor chips to be tested, usually have a square or rectangular shape. Accordingly, the forward contact tips of the probes are necessarily arranged in a square or rectangular pattern corresponding to the arrangement of a multiplicity of electrodes (i.e. bonding pads) formed on the periphery of each semiconductor chip. When the probes are fixed in a conical radial array with a given acute angle relative to the printed substrate, as in the above mentioned conventional probe board, the lengths of the free ends of the test probes from the fixed positions to the respective forward tips differ from one another, and the vertical distances from the respective forward end portions of the probes to the face of the printed substrate differ from one another.
Namely, if the given acute angle is represented by .theta., and the difference in the lengths of the free ends is represented by .DELTA.l, the difference .DELTA.h in the vertical distances is given by .DELTA.h=.DELTA.l sin .theta.. When the acute angle .theta. is set to be 9.degree., the difference .DELTA.h in the vertical distances becomes to be 78 .mu.m for every 0.5 mm difference .DELTA.l in the lengths of the free ends. Accordingly, the difference .DELTA.h in the vertical distances between a probe having a free end of a length l of 4 mm and a probe having a free end of a length l of 6 mm becomes to be as great as 313 .mu.m.
In assembling the above probe board, firstly the probes are arranged to form the above mentioned conical radial array, and with use of an assembling tool having a positioning mask with a multiplicity of small holes corresponding to the electrodes to be tested, the forward tips of the probes are inserted into the small holes of the mask. In this state, the probes are fixed to a support base, Finally, the support base is attached to a printed substrate. Accordingly, in the assembled probe board, the forward contact tips of the probes have variations in the above mentioned vertical distances. Therefore, there is a serious disadvantage that it is then required to adjust the probes to bring their forward contact tips to a common plane with an accuracy of .+-.25 m. Such an adjustment is usually done manually by observation with naked eyes.
As the forward end portions of the probes are bent in a vertical direction, it is conceivable that the variations in the vertical distances may be corrected by adjusting the lengths of the forward ends from the bent portions to the respective forward tips. However, such a concept is not practical in that a number of different kinds of probes having minutely differing lengths of the forward ends from the bent portions to the forward tips will have to be prepared and will have to be properly selected at the time of assembling the probes.
In the above mentioned case, even when the adjustment is properly done by bringing the forward contact tips to a common plane, the lengths of the probes from the fixed portions to the respective forward ends differ from one another, and consequently, when the forward contact tips are pressed on the electrodes of a conductor chip to be tested, the contacting pressures imparted to the electrodes differ from one probe to another. The variations in the contacting pressures of the probes mean corresponding variations in the stress imparted to the probes, which in turn give rise to variations in the residual strain in the probes, thus leading to variations in the durability of the probes. Consequently, readjustment of the probes to correct the deviation of the forward contact tips from a common plane is frequently required, thus leading to shortening of the useful life of the probe board.
Further, the deviation of the forward contact tips from the common plane and the variations in the contacting pressures due to the differences in the lengths of the probes, bring about a so-called piezo effect whereby the crystal structure of the wafer chip is changed by uneven pressures imparted thereto, and consequently, the characteristics of the semiconductor elements will be changed. Thus, not only the wafer chip tested is thereby adversely affected, but also the test result thereby obtained will be unreliable.